Method for potential controlled electroplating of fine patterns on semiconductor wafers

ABSTRACT

Controlled-potential electroplating provides an effective method of electroplating metals onto the surfaces of high aspect ratio recessed features of integrated circuit devices. Methods are provided to mitigate corrosion of a metal seed layer on recessed features due to contact of the seed layer with an electrolyte solution. The potential can also be controlled to provide conformal plating over the seed layer and bottom-up filling of the recessed features. For each of these processes, a constant cathodic voltage, pulsed cathodic voltage, or ramped cathodic voltage can be used. An apparatus for controlled-potential electroplating includes a reference electrode placed near the surface to be plated and at least one cathode sense lead to measure the potential at points on the circumference of the integrated circuit structure.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/515,414, filed on Feb. 29, 2000, and now abandoned.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to electroplating methods and apparatusand, in particular, to processes and apparatus for potential-controlledelectrodeposition of metal into small integrated circuit features suchas vias and trenches.

2. Description of Related Art

Conductive interconnections on integrated circuits typically take theform of trenches and vias. In modern submicron integrated circuits,trenches and vias are typically formed by a “damascene” or “dualdamascene” process as described, for example, in the reference ULSITechnology, Eds. C. Y. Chang and S. M. Sze (McGraw-Hill, 1996, pp.444-445.) In damascene processing, an interlayer dielectric (typicallySiO₂) is deposited atop a planarized layer containing, for example, ametal via. The top dielectric layer is patterned and etched, typicallyusing conventional photolithographic procedures. Metal is then depositedinto features and on the flat field region atop the features, typicallyfirst by CVD, PVD and then by electrodeposition. The metal layer istypically planarized resulting in the desired metallic pattern. Dualdamascene processing is similar but makes use of two patterning andetching steps and typically fills features with metal spanning more thanone layer in a single metallization step. A more complete description ofdamascene and dual damascene processing is found in the cited reference.

As the art moves towards integrated circuits having reduced featuresizes, it becomes increasingly difficult to form electrically conductivemetallizations such as vias, contacts and conductors. Techniques forforming such metallizations include physical vapor deposition (PVD),chemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD) and electrochemical deposition (also referred to aselectroplating or electrodeposition) of metals such as copper.Electroplating is particularly well suited for the formation of smallembedded damascene feature metallization due to the ability to readilycontrol the growth of the electroplated film for bottom-up fillingwithout voids, and due to the superior electrical conductivitycharacteristics of the electroplated film. However, there are alsoseveral obstacles which need to be overcome to fully realize theseadvantages.

One challenge facing damascene and dual damascene processing techniquesis the difficulty of initiating the growth of the metal film withinrecessed features without forming voids or seams. In typical PVD andsome CVD processes, metal may preferentially deposit near the top ofrecessed features leading to a “bottleneck” shape. Further plating ofmetal onto the bottleneck may result in sealing the top of the featurebefore completely filling the feature with metal, creating a void. Voidsincrease the resistance of the conductor over its designed value due tothe absence of planned-for conductor. Also, trapped electrolyte insealed voids may corrode the metal. This may lead to degraded deviceperformance or device failure in extreme cases.

The problem of electroplating metal into integrated circuit featureswithout the formation of voids or seams is the subject of previouslyfiled U.S. patent application Ser. No. 09/410,170, commonly assignedwith the present invention, (hereinafter “prior application”), and nowabandoned and incorporated herein by reference. The prior applicationidentified previously unrecognized problems with conventionalelectroplating techniques and described a four-step approach to overcomethem or to mitigate their deleterious effects on the electroplatingprocesses. The four phases relate to the following: 1) the entry of thewafer into the electroplating bath, including procedures to reduce oreliminate corrosion of thin metal seed layers already deposited on thesurfaces of recessed features (“entry phase”); 2) the conformal growthof metal seed layers having sufficient thickness to support delivery ofcurrent to the bottom of the feature and permit subsequent bottom-upfeature filling (“initiation phase”); 3) preferential deposition ofmetal onto the bottom of high aspect ratio (“AR”) features (having AR'sgreater than about 0.5) leading to bottom-up filling and the reductionof the ARs; and 4) rapid filling of low AR features in a rapidsubstantially conformal manner until the required metal thickness isachieved.

The methods described in the prior application relate generally toconstant current (“galvanostatic”) or controlled current(“galvanodynamic”) processes. Thus, the current was the parametercontrolled and the voltage was understood to take on whatever valuenecessary to achieve the specified current. Use of galvanostatic orgalvanodynamic processes has the advantage of requiring typicallysimpler and less expensive power sources than those typically requiredin the practice of controlled-voltage (“potentiodynamic”) processes.

Prior art and literature pertaining to potentiostatic andpotentiodynamic electroplating is limited in comparison with theliterature and prior art related to galvanostatic and galvanodynamicelectroplating. Controlled-potential electroplating is typically a morecomplicated process than controlled current electroplating for severalreasons. 1) Controlled-current power supplies are more readily availableat lower cost while only a limited number of (typically) more expensivepower supplies are available for controlled-potential service. 2) A“four wire” system is typically required in controlled-potentialelectroplating in order to compensate adequately for potential drop incurrent-carrying wires and for contact resistances. For applicationsrequiring excellent metal thickness uniformity, such as the integratedcircuit fabrication processes described here, a reference electrodeprobe is advisable.

The present invention describes several advantages to be obtained withthe use of controlled-potential electroplating processes for smallrecessed features in integrated circuit devices. Equipment fully to takeadvantage of potentiodynamic electroplating processes for the four-stepfilling of integrated circuit features with metal is also described.

SUMMARY OF THE INVENTION

Controlled-potential electroplating provides an effective method ofelectroplating metals onto the surfaces of high aspect ratio recessedfeatures of integrated circuit devices. The rate of electroplating isstrongly dependent on current density which is correlated with theapplied voltage. Thus controlled-potential methods provide tightercontrol of the electroplating process than controlled-current methods.In addition, controlled-potential electroplating methods are moreuniversally applicable, from wafer to wafer, essentially independent ofthe number, size, and distribution of recessed features on the wafer,than are controlled-current methods.

Controlling the potential during electroplating provides a process tomitigate corrosion of a metal seed layer on recessed features in a layerof an integrated circuit structure. The corrosion is due to contact ofthe seed layer with an electrolyte solution during the entry phase. Thepotential can also be controlled to provide conformal plating over theseed layer during the initiation phase and bottom-up filling of therecessed features during the third, bottom-up filling phase. At eachphase, a constant cathodic voltage, pulsed cathodic voltage, or rampedcathodic voltage can be used.

An apparatus for controlled-potential electroplating includes areference electrode placed near the surface to be plated and at leastone cathode sense lead to measure the potential at points on thecircumference of the integrated circuit structure.

BRIEF DESCRIPTION OF THE DRAWINGS

All drawings are schematic and not to scale. In the figures, similarreference numbers are used to identify similar elements.

FIG. 1 is a schematic diagram of an electroplating apparatus in whichembodiments of the present invention are practiced.

FIG. 2 is a top view of the clamshell of the electroplating apparatus.

DETAILED DESCRIPTION

The present invention is directed towards the controlled-potential(“potentiodynamic”) plating of metals onto the surfaces of high aspectratio (“AR”) features as would typically be encountered in thefabrication of integrated circuits. “Aspect Ratio” of a feature isdefined as the ratio of the feature's depth to its width, in which widthis determined in the plane of the wafer. For a trench, the width refersto the smaller dimension in the plane of the wafer. For a via, width isthe feature diameter or side length in the plane of the wafer.

Accurate Measurement of Wafer Potential

It is relatively simple to measure and control the voltage (potential)across the electroplating cell; that is, the voltage between a workingelectrode (cathode) and a counter electrode (anode). However, attemptingto use a voltage value determined at the counter electrode to controlthe electroplating process is subject to considerable uncertainty.Interpreting the voltage as measured at the counter electrode iscomplicated by the potential drop across the counter electrode caused bysurface kinetics and diffusion (concentration) potential drops acrossthe anode/electrolyte interface. Such potential drops can be complicatedfunctions of current, hydrodynamic effects, counter electrode size,shape, age, etc. as well as on the processing history and the timeduring the electroplating process at which the voltage at the counterelectrode is measured. Therefore, using a polarized electrode such asthe counter electrode for controlling the potential in a potentiodynamicelectrodeposition process is a difficult task in practice.

FIG. 1 is a schematic depiction of a typical apparatus forpotentiodynamic electroplating according to one embodiment of thepresent invention. FIG. 1 depicts a “four wire” electroplating system astypically employed in the practice of the present invention. Anelectroplating system 1 contains a electrolyte solution or bath 2 aswell as a wafer 8 to be electroplated. As described in the priorapplication, typical electrolyte baths contain ions of the metal to beplated and additives that may be plating suppressors, such as largemolecules, or accelerator additives that adsorb onto surfaces and canincrease the electroplating current locally (microscopically) in thepresence of suppressors. As described below, the details of the initialcontact between the wafer 8 and the electrolyte solution 2 is a probablesource of damage to the metal seed layer previously deposited on thewafer and a complicating factor in electroplating high AR features.

FIG. 1 depicts wafer 8 as a multilayer structure. Surface 3 denotes themetal seed layer previously deposited onto wafer 8 by PVD or othermeans. Wafer 8 may be comprised of several layers of semiconductor,dielectric and other metallic layers underlying seed layer 3. Wafer 8 isdepicted with features of varying aspect ratio, such as feature 16 witha large AR and feature 17 with a small AR. Field region 18 is the flatpart of the surface of wafer 8 between features. The specific structureof the wafer 8 in layers below the seed layer 3 is not an importantcharacteristic in the practice of the present invention except as suchlayers affect the feature structure of layer 3 or may be themselvesexposed to the electrolyte solution.

Wafer 8 is held in the electroplating bath by a wafer-sealing“clamshell,” 20, shown in greater detail in FIG. 2. Clamshell 20includes contact fingers 22, which are electrically connected to anexternal power supply 60 (connection not shown). Contact fingers 22 aretypically grouped in quadrants 25. For example, for electroplating a 200mm wafer, 128 contact fingers may be grouped in four quadrants of 32contact fingers, each. When wafer 8 is inserted in clamshell 20 fromabove, contact fingers 22 contact the edge of the wafer, which turnswafer 8 into the working electrode. Electroplating of metals involvesthe deposition of metallic cations from the electrolyte solution. Theworking electrode is thus a cathode and a first wire of the “four wiresystem.”

Also depicted on FIG. 2 is cathode sense lead 5 that does not carry anyappreciable current. The function of cathode sense lead 5 is to sensethe voltage at one or more points directly on wafer 8 rather than relyon voltage determined at the power supply as is conventionally done inpotentiodynamic electroplating. Cathode sense lead 5 comprises anotherof the “wires” of the four wire system. The use of such a lead makingdirect contact with the surface of the wafer 8 is important in makingaccurate potential measurements. The cathode sense lead 5 is not subjectto appreciable voltage drop between the contact point on the surface ofthe wafer and the control measurement point of an operational amplifierwhich is responsible for the control of the potential in the presentpotentiodynamic electroplating system.

As shown in FIG. 2, the cathode sense leads can be incorporated into theclamshell design, for example, into the Novellus Systems Sabre™ andSabre™ apparatus as a modification thereof In the particular depictionin FIG. 2, cathode sense lead 5 includes two separated, electricallyisolated fingers 15 contacting the wafer circumference. The contactfingers span from about 5° to about 10° around the circumference ofwafer 8. From 1 to 4 contact “fingers” may be used. Additional cathodesense leads can be added, placed preferably at 90° or 180° from theprimary cathode sense lead. These secondary cathode sense leads can beused to measure any non-uniformity of potential in the wafer surface aswell as breaks occurring in wafers as they are loaded into the clamshell.

The counter electrode (anode) is depicted as 7 in FIG. 1 and may beconsidered another wire of the four wire system. In the electroplatingof metals, anode 7 is typically a source of metal cations to replenishthe cations of electrolyte solution 2 as they are deposited onto surface3. Anode 7 may be a copper metal electrode for the electrodeposition ofCu. Alternatively, anode 7 is a dimensionally stable metal anode such asa platinum or titanium anode, as known in the art.

The electroplating apparatus also includes a reference electrode 6,which is used to monitor and control the potential of the wafer. Thereference electrode measures the potential of the electrolyte solutionwith respect to the surface of the wafer 3. The potential measured bythe reference electrode 6 differs from the potential measured by cathodesense lead 5 due to the charge-transfer potential drop across thesurface of wafer 8 and the ohmic drop in solution between the wafersurface and the probe location due to diffusion overpotentials(concentration effects). Reference electrode 6 is preferably positionedin the electrolyte between the wafer and the counter electrode and asclose as practical to the working electrode (wafer).

A reference electrode is preferably a probe electrode carryingessentially no current. That is, a reference electrode is connected to ahigh impedance circuit and is, itself, unpolarized. Several differentreference electrodes meet these criteria and are usefull in the practiceof the present invention. A few examples are Hg/Hg₂Cl₂ (calomel),Ag/AgCl, Hg/HgSO₄, or a copper metal electrode (Cu/Cu⁺² couple). Acopper metal electrode in a separate chamber filled with electrolyteoffers several advantages as the reference electrode in theelectroplating of copper, an important procedure in damascene and dualdamascene processes. Among these advantages are the five nowitemized. 1) A copper metal reference electrode will not be contaminatedby the diffusion of Cu⁺² ions from the electroplating solution into thereference electrode chamber. 2) A copper metal reference electrode willnot contaminate the bath by diffusion of ions from the referenceelectrode chamber into the electrodeposition bath (as would be the casefor the Ag/AgCl reference electrode, for example, changing the Cl⁻concentration in the electrodeposition bath). 3) A copper metalreference is more electrically stable than electrodes having liquidjunctions due to the absence of the formation of a liquid junctionpotential. 4) A copper reference electrode simplifies the processcontrol in the electrodeposition of copper as such an electrode alwayshas a potential of 0 volts with respect to the working electrode. 5) Acopper metal reference electrode is easy to maintain and inexpensive.The criteria for selection of reference electrodes are described, forexample, in John S. Newman, Electrochemical Systems, (Prentice-Hall,Inc., Englewood Cliffs, N.J., 1973) pgs. 107-123.

For the electrodeposition of copper, the reference electrode ispreferably a copper metal electrode contained in a capillary chamberflooded with electrolyte. Reference electrode 6 in FIG. 1 consists of acapillary chamber containing a metal electrode. The electrolyte in thereference electrode chamber contains the same metal ions as theelectrolyte in the electrodeposition bath but does not containaccelerator or suppressor additives. It is also advantageous in thepractice of the present invention that the chamber containing the copperelectrode be separated from the plating chamber by insulating walls madeof plastic or glass or other material resistant to the electroplatingsolution, or by diffusion barriers such as porous glass frittedmaterial. It is also found favorable in the practice of the presentinvention that the capillary chamber containing the copper metalelectrode have its opening near the wafer and terminating in closeproximity to the wafer.

A plurality of reference electrodes may be utilized, preferably havingcapillary outlets in the electroplating solution at various positionsnear the wafer. These multiple reference electrodes can be used tomonitor the potential in the electroplating solution at various pointsin proximity to the wafer surface and can be used in a feedback manner.Thus, they can provide important information in controlling theelectrodeposition process to achieve and maintain plating uniformityfrom the center of the wafer to its edges. Multiple reference electrodescan also be useful in designing appropriate shielding or in dynamicallymodifying the shape of shielding members during the plating process.Dynamic shielding is described, for example, in the U.S. Pat. No.6,402,923 of Mayer et al., issued on Jun. 11, 2002, entitled “Method andApparatus for Uniform Electroplating of Integrated Circuits Using aVariable Field Shaping Element,” which is commonly assigned with thepresent application and incorporated herein by reference.

Potentiodynamic Electroplating

In accordance with the present invention, a method of electroplating ispresented which is particularly well suited for electroplating smallfeatures having high AR. Additionally, embodiments of the presentinvention relate to processes of electroplating a substrate havingfeatures with various ARs in a manner efficiently producing good qualitycoating for all such features.

The present invention typically includes four distinct phases designedto reduce or eliminate one or more problems commonly associated withconventional electroplating of recessed features.

An initial PVD deposition typically results in thin metal seed layers onthe walls and bottoms of trenches and vias. As an example,electroplating of copper onto a tantalum (Ta) and/or tantalum nitride(TaN) barrier layer on a wafer is described here. However, theprocedures and methods described are applicable to other platingprocesses, as apparent to those of ordinary skill in the art.

In addition to being thin, the morphology of the seed layer coating onwalls/bottoms of trenches/vias can be non-uniform, consisting ofisolated islands of copper lying on an underlying layer of tantalum (orother material). If exposed to oxygen or water, a tantalum oxide layermay form. The inventors have observed that placing a substrate havingsuch islands into an electrolyte plating bath generally exacerbateswall/bottom non-uniformity, likely due to spontaneous electrochemicaletching. Thus, the first phase, referred to as “the entry phase,”includes procedures to reduce or avoid etching and corrosion of thecopper seed layers within the features and thereby maintain theintegrity of the seed layer for subsequent phases.

As the first, entry, phase avoids exacerbating the wall/bottomnonuniformity, the second or “initiation phase” repairs the survivingnon-uniformity in the recessed features and thickens the metallic seedlayer. The initiation phase includes nucleation and growth resulting inisland-bridging of the seed layer non-uniformity within the features.During the initiation phase, the seed layer within the features isoverlaid with another layer that is more uniform and provides sufficientelectrical conductivity to the walls and bottoms of features to allowthe selective bottom-up filling procedures of the subsequent phase(phase 3) to be carried out.

The features next must be filled with conductive material in such amanner as to reduce or eliminate the formation of seams and/or voids. Itis found that features having ARs typically in excess of 0.5 (“high AR”)require different electroplating conditions from those having low ARs(less than approximately 0.5) in order to achieve electroplating withseam/void reduction (or elimination). It is useful to describe thepresent invention in terms of the different conditions required forfilling high and low AR features as two separate phases. However, ashigh AR features fill according to the desired bottom-up manner, thefeatures' AR decreases. In actual practice, the electroplatingconditions can be changed in a continuous manner to achieve filling offirst high then low AR features. However, to make the discussion moredefinite, we consider the filling of high AR features separate fromfilling of low AR features.

The filling of high AR features, according to the present invention,referred to as “the bottom-up electrofilling phase,” involves thedeposition of electrically conductive material preferentially onto thebottoms of the high AR features. The formation of seams and voids isreduced or avoided for features of all sizes and ARs in this phase. Thefourth phase, referred to as “the low AR feature filling phase,”involves the electroplating of electrically conductive material into lowAR features, typically with an AR less than 0.5.

1. The Entry Phase

During the initial or “entry phase” of the electroplating process forULSI wafers, the existing metallic seed layer within small features issubject to corrosion reactions, crystal redistribution and, in general,removal of seed material from the bottom regions of trenches and vias.Either current or voltage can be fixed at a set value during this phaseat a sufficiently low value to reduce the tendency for corrosion anddissolution of the seed layer.

If the current is fixed while the wafer is introduced into theelectroplating solution, some portion of the wafer (for example, thecenter) will become wet with electrolyte solution prior to otherportions of the wafer surface. The wetted portion of the wafer is theonly region on the surface of the wafer subject to a complete electricalcircuit. Thus, the amount of wafer surface area initially connected tothe circuit is initially significantly less than that of the total wafersince the entire wafer surface does not become wetted instantaneously.Under conditions of controlled current, the wetted region will see amuch higher current density than the portion of the wafer that becomeswet somewhat later. The difference in current density can cause adifference in feature filling rates at different parts of the wafer,e.g. too high a current can result in voids. In contrast, if the voltageis fixed to a given value during the entry phase, the current suppliedto the wafer will slowly increase as the fraction of the wafer that iswetted increases, but the current density will remain constant. Fixingthe wafer potential has the effect of fixing the current density and theoverall current supplied. Using fixed wafer potential, the only effectof the time dependent wetting is a slight difference in total time ofexposure to the electrolyte solution. Feature filling, which is stronglydependent on current density, is essentially uniform at all positions onthe wafer surface.

Various process parameters have been investigated by the presentinventors for the entry phase using a controlled voltage method. It hasbeen determined that application of the following voltages to the wafersurface immediately upon first contact with the electrolyte solutionprovide good process performance.

EXAMPLE A: Application of a constant cathodic dc voltage having a valuefrom about −5 mV (millivolts) to about −100 mV with respect to areference electrode, for a duration of about 0 to about 5 seconds. Forelectroplating of copper, a copper metal reference electrode ispreferred.

EXAMPLE B: Application of a pulsed, cathodic voltage having a value fromabout −10 mV to about −500 mV with respect to a reference electrode,having a waveform period from about 0.1 ms (milliseconds) to about 10 msand a duty cycle from about 1% to about 50%, for a duration of about 0to about 5 seconds.

2. The initiation phase.

The purpose of the initiation phase (phase 2) is to nucleate and grow arelatively thin film (less than approximately 300 Å) in a substantiallyconformal manner onto the recessed regions of the surface. The recessedsurface regions typically have a poor quality metallic seed layer film(islands or spotty) following the entry phase. That is, even though theentry phase avoids or mitigates damage to the seed layer upon initialcontact with the electrolyte solution, the seed layer is still typicallyinadequate to support sufficient current for the bottom-up filling ofphase 3. During the initiation phase, metal is growing conformallythicker on the walls of the features and in the field region at the samerate. Such conformal growth actually decreases the active(current-carrying) surface area of the wafer within the features.

Thus, for a controlled-current mode of plating, the total currentpassing through the wafer is the same, but the surface area within thefeature is decreasing and the local current density within the featureis increasing, which will cause the cell voltage to increase. Thisincrease in current density may result in the features filling tooquickly. As explained in the prior application, low current densityleads to slow plating (as compared to the rate of diffusion of copperand additives to the surface) resulting in no preference for copper orsuppression additives for either the top or bottom of recessed features,and hence, conformal plating. At higher current density, depletion ofsuppression additives preferentially at the bottom of recessed featuresresults in bottom up filling which is desired at the following, thirdphase, but not at the initiation phase. Further, an increase in currentdensity may have a deleterious effect because too rapid consumption ofadditives in the electrolyte bath might reduce the positive effect ofenhanced film nucleation during the initiation phase of plating.

When a controlled-potential plating process is used, the current densityis expected to remain relatively constant throughout the initiationphase of plating. (Note, however, that the thickness of the entiremetallic field region, thus of the entire wafer, is increasing duringthe initiation phase, so that the voltage drop across the thicker fieldseed layer will be somewhat less than the corresponding voltage drop atthe start of the initiation phase.)

Typical methods for the controlled-potential initiation phase to providesubstantially conformal electroplating on the field region and on therecessed features of the surface are as follows:

EXAMPLE C: Apply a constant cathodic dc voltage from about −5 mV toabout −100 mV with respect to a reference electrode. Application of theconstant dc cathodic voltage is preferably carried out fromapproximately 0 to 30 seconds.

EXAMPLE D: Apply a pulsed cathodic voltage of from approximately −10 mVto approximately −1000 mV having a waveform period of about 0.1 to 10 msand a duty cycle of approximately 1% to 50%. A background cathodic dcvoltage of from about −5 mV to about −100 mV may be optionallysuperimposed on the pulsed cathodic voltage.

EXAMPLE E: Apply a ramped voltage starting at about −5 mV to about −100mV and varying to about −100 mV to about −500 mV with a ramp speed ofapproximately −1 mV/sec to approximately −100 mV/sec (increasing themagnitude of the negative voltage, decreasing the voltage, at the ratesindicated).

We note that the use of a proper shape and length of pulsed potentialwaveform during the initiation phase has the advantage of forcing mostof the overall cell potential drop to occur between the wafer surfaceand the solution immediately adjacent to the surface. If the potentialis stepped quickly to avoid “mass-transfer-control” effects, the currentdensity is controlled by charge transfer both locally and globallyacross the wafer surface (see, for example, A. J. Bard and L. R.Faulkner, “Electrochemical Methods, Fundamentals and Applications,” JohnWiley and Sons, p. 141 (1980)). Mass-transfer-control as used aboverefers to an electroplating process in which the plating rate iscontrolled by the rate at which the plating cation migrates through theelectrolyte solution and reaches the surface to be plated.Mass-transfer-control is undesirable because the bottom and sidewalls ofhigh AR features would plate at substantially different rates. Thus,during the initiation phase a conformal deposition is expected to occurat every location within small features.

The present inventors have also observed that another means of improvingconformal plating of the bottom and sidewalls of small features whilesimultaneously reducing the plating thickness at the upper part of thefeature (effectively widening the feature towards its mouth) is to use awaveform having a low anodic dc voltage superimposed on the pulsedcathodic waveform. During the period of the waveform lacking a cathodicpulse, there is a constant lower dc voltage and lower-current anodicsteady state. Thus, the dissolution of metal back into the plating bathis primarily governed by geometric effects whereby the top, sharp edgesof the features are typically removed faster due to their greaterexposure to solution. Additives in the plating bath will adsorb todifferent degrees over the surface of the features and tend to alterthis geometric leveling. Thus, the magnitude of the anodic voltage(related to the anodic and cathodic current densities and the relativeon and off times-duty cycle) will require adjustment for the type andamount of additives in the electrolyte solution.

3. The Bottom-up Filling Phase.

It was shown in the prior application that at somewhat higher currentsthan those used in the initiation phase, the electrodeposition ratebecomes sufficiently high that current-suppressing additives cannot bereplenished within the diffusion-inaccessible feature as such additivesare electroreduced or incorporated into the deposited layer. Thisdestruction of current suppressers leads to acceleration of theelectroplating rate within the features in comparison with the fieldregion, that is the flat region on the wafer surface between features.Thus, there appears to be a certain minimum current density or fillingrate that is required in order to establish the necessary degree ofplating selectivity between the feature and the field. The platingselectivity, which may be termed “superfilling,” is responsible forbottom-up filling.

As the pattern density, or number of features per unit area on thewafer, increases, the total current that must be supplied to the waferto superfill the features must be increased. In addition, the necessaryminimum local-current-density for superfilling also depends on thefeature aspect ratio, width, and shape.

According to an aspect of the present invention, controlling thepotential during electroplating provides an effective method ofcontrolling the plating rate. Control of the potential can be moreuniversally applicable, from wafer to wafer, where the wafers havedifferent numbers of features, sizes of features, and distribution offeatures, than is control of the current. Controlled-potentialelectroplating achieves bottom-up filling, essentially independent ofthe number, size, and distribution of features on the wafer. Thecontrolled-potential electroplating method for the bottom-up fillingphase can use a constant, fixed potential. Alternatively, the controlledpotential electroplating method uses a ramped potential in which thepotential is increased with time. For a given desired potential timeprofile (fixed or ramped), the power supply driving the electroplatingprocess is operated to compensate for any variation in feature densityby adjusting the current to maintain the potential at the specifiedvalue.

It has also been shown that current-controlled pulse plating iseffective in the bottom-up filling phase for removing “bottlenecked”structures that may occur. A reverse pulse of current reverses theplating process and removes metal from the surfaces. In particular, thereverse pulse is useful to remove material building up near the openingof features, leading to a bottleneck.

According to another aspect of the present invention, an increasingpotential is combined with potential reversing to provide an effectivebottom-up fill without bottlenecking. The reverse potential is appliedfor a time period sufficient to remove any bottlenecked structures inthe upper periphery of the features, but insufficient to remove anyappreciable amount of metal from the bottom of the features. Using anyof the controlled-potential techniques, constant voltage, rampedvoltage, or ramped voltage with potential reversing, the bottom-upfilling phase is continued until all features initially having aspectratios greater than about 0.5 are filled.

The precise adjustments of controlled-potential bottom-up filling willdepend on the particular choice of additives and electrolyte bathconditions. However, the following examples have given satisfactoryresults in Shipley Ultrafill™ (Marlboro, Conn.) plating bath.

EXAMPLE F: A constant voltage from approximately −50 mV to approximately−750 mV with respect to a reference electrode is applied.

EXAMPLE G: An increasing voltage is used beginning at the final voltageof the initiation phase (phase 2) and ending at an increased (negative)voltage of about −100 mV to about −1000 mV. The precise values depend onwafer size, feature size and density.

EXAMPLE H: A pulsed cathodic voltage from approximately −100 mV toapproximately −1000 mV, having a waveform period from about 0.1 ms toabout 10 ms and a duty cycle from about 1% to about 50%, is applied.

EXAMPLE I: A pulsed voltage as in EXAMPLE H is superimposed over abackground dc anodic voltage.

EXAMPLE J: Applying an ac voltage increasing in frequency from aninitial frequency of about 1 Hz to a final frequency of about 100 kHz.This procedure allows the additives initially to adsorb more completelyover the surfaces of the vias and trenches while later improvingbottom-up filling by faster consumption of additives near the bottom ofthe features.

4. Low AR Filling Phase.

In addition to previously high AR features, very small AR features mayoccur on integrated circuits (e.g. “pads”). It is the function of thefourth phase to fill such features as well as to finish the filling ofthose features whose filling has begun in previous phases, if necessary.This generally is done by substantially conformal filling since theelectroplating process is typically governed by electric field anddiffusion dependent mechanisms during which additive depletion or sidewall closure is not likely. Therefore, a layer of metal approximatelyequal to the dielectric layer in thickness is generally deposited(typically between 0.7 and 1.4 μm) to complete the electroplatingprocess. As the fourth phase is generally the longest processing step,it is desirable to deposit metal uniformly using as high a currentdensity as possible, for example, a current density of 25 mA/cm². Thishigh current density can be achieved by maintaining a high constantvoltage or a high constant current.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific and preferred embodimentsillustrated and described. Rather, it is intended that the scope of theinvention be determined by the appended claims.

We claim:
 1. A method of electroplating a metal onto a surfacecomprising a field region and a plurality of recessed features, therecessed features having a range of sizes, the surface having a metalseed layer, the method comprising: contacting the surface with anelectroplating solution comprising copper metal ions and at least oneadditive; applying a controlled first voltage to the surface for a timesufficient to provide a substantially conformal thin conductive layer onthe recessed features, the first voltage being from about −5 millivoltsto about −100 millivolts with respect to a copper metal referenceelectrode disposed in a reference solution comprising copper metal ionsbut not the additive; applying a controlled second voltage to thesurface, wherein the second voltage is larger than the first voltage,the second voltage controlled such that electroplating occurspreferentially on the bottoms of the recessed features, wherein the rateof electroplating is essentially independent of the number of recessedfeatures, the size of the recessed features, and the distribution of therecessed features on the surface; and increasing said second voltage toa value providing a condition of rapid conformal plating.
 2. The methodof claim 1, wherein the first voltage comprises a constant cathodicvoltage.
 3. The method of claim 1, wherein the first voltage is appliedfor a period of no greater than about 30 seconds.
 4. A method ofelectroplating a metal onto a surface comprising a field region and aplurality of recessed features, the recessed features having a range ofsizes, the surface having a metal seed layer, the method comprising:contacting the surface with an electroplating solution comprising coppermetal ions and at least one additive; applying a controlled firstvoltage to the surface for a time sufficient to provide a substantiallyconformal thin conductive layer on the recessed features, the firstvoltage being from about −10 millivolts to about −1000 millivolts withrespect to a copper metal reference electrode disposed in a referencesolution comprising copper metal ions but not the additive; applying acontrolled second voltage to the surface, wherein the second voltage islarger than the first voltage, the second voltage controlled such thatelectroplating occurs preferentially on the bottoms of the recessedfeatures, wherein the rate of electroplating is essentially independentof the number of recessed features, the size of the recessed features,and the distribution of the recessed features on the surface; andincreasing said second voltage to a value providing a condition of rapidconformal plating.
 5. The method of claim 4, wherein the first voltagecomprises a pulsed cathodic voltage.
 6. The method of claim 5, whereinthe first voltage has a duty cycle of from about 1% to about 50% and apulse length of from about 0.1 millisecond to about 10 milliseconds. 7.The method of claim 5, wherein the first voltage further comprises abackground cathodic dc voltage of from about −5 millivolts to about −100millivolts.
 8. A method of electroplating a metal onto a surfacecomprising a field region and a plurality of recessed features, therecessed features having a range of sizes, the surface having a metalseed layer, the method comprising: contacting the surface with anelectroplating solution comprising copper metal ions and at least oneadditive; applying a controlled first voltage to the surface for a timesufficient to provide a substantially conformal thin conductive layer onthe recessed features, the first voltage being varied from a value fromabout −5 millivolts to about −100 millivolts to a value from about −100millivolts to about −500 millivolts with respect to a copper metalreference electrode disposed in a reference solution comprising coppermetal ions but not the additive; applying a controlled second voltage tothe surface, wherein the second voltage is larger than the firstvoltage, the second voltage controlled such that electroplating occurspreferentially on the bottoms of the recessed features, wherein the rateof electroplating is essentially independent of the number of recessedfeatures, the size of the recessed features, and the distribution of therecessed features on the surface; and increasing said second voltage toa value providing a condition of rapid conformal plating.
 9. The methodof claim 8, wherein the first voltage is continuously varied.
 10. Themethod of claim 8, wherein the first voltage is continuously varied at arate from about −1 millivolt/second to about −100 millivolt/second. 11.A method of electroplating a metal onto a surface comprising a fieldregion and a plurality of recessed features, the recessed featureshaving a range of sizes, the surface having a metal seed layer, themethod comprising: contacting the surface with an electroplatingsolution comprising copper metal ions and at least one additive;applying a controlled first voltage to the surface for a time sufficientto provide a substantially conformal thin conductive layer on therecessed features, the first voltage having a value sufficient tominimize dissolution of the seed layer; applying a controlled secondvoltage to the surface, wherein the second voltage is from about −50millivolts to about −750 millivolts with respect to a copper metalreference electrode disposed in a reference solution comprising coppermetal ions but not the additive, the second voltage controlled such thatelectroplating occurs preferentially on the bottoms of the recessedfeatures, wherein the rate of electroplating is essentially independentof the number of recessed features, the size of the recessed features,and the distribution of the recessed features on the surface; andincreasing said second voltage to a value providing a condition of rapidconformal plating.
 12. The method of claim 11, wherein the secondvoltage comprises a constant cathodic voltage.
 13. A method ofelectroplating a metal onto a surface comprising a field region and aplurality of recessed features, the recessed features having a range ofsizes, the surface having a metal seed layer, the method comprising:contacting the surface with an electroplating solution comprising coppermetal ions and at least one additive; applying a controlled firstvoltage to the surface for a time sufficient to provide a substantiallyconformal thin conductive layer on the recessed features, the firstvoltage having a value sufficient to minimize dissolution of the seedlayer; applying a controlled second voltage to the surface, wherein thesecond voltage is varied from a value of the first voltage to a valuefrom about −100 millivolts to about −1000 millivolts with respect to acopper metal reference electrode disposed in a reference solutioncomprising copper metal ions but not the additive, the second voltagecontrolled such that electroplating occurs preferentially on the bottomsof the recessed features, wherein the rate of electroplating isessentially independent of the number of recessed features, the size ofthe recessed features, and the distribution of the recessed features onthe surface; and increasing said second voltage to a value providing acondition of rapid conformal plating.
 14. The method of claim 13,wherein the second voltage is continuously varied.
 15. A method ofelectroplating a metal onto a surface comprising a field region and aplurality of recessed features, the recessed features having a range ofsizes, the surface having a metal seed layer, the method comprising:contacting the surface with an electroplating solution comprising coppermetal ions and at least one additive; applying a controlled firstvoltage to the surface for a time sufficient to provide a substantiallyconformal thin conductive layer on the recessed features, the firstvoltage having a value sufficient to minimize dissolution of the seedlayer; applying a controlled second voltage to the surface, wherein thesecond voltage is from about −100 millivolts to about −1000 millivoltswith respect to a copper metal reference electrode disposed in areference solution comprising copper metal ions but not the additive,the second voltage controlled such that electroplating occurspreferentially on the bottoms of the recessed features, wherein the rateof electroplating is essentially independent of the number of recessedfeatures, the size of the recessed features, and the distribution of therecessed features on the surface; and increasing said second voltage toa value providing a condition of rapid conformal plating.
 16. The methodof claim 15, wherein the second voltage comprises a pulsed cathodicvoltage.
 17. The method of claim 16, wherein the second voltage has aduty cycle from about 1% to about 50% and a pulse length from about 0.1millisecond to about 10 milliseconds.
 18. The method of claim 16,wherein the second voltage further comprises a background dc anodicvoltage.